1. Field of the Invention
This invention relates generally to an N-face GaN semiconductor device and, more particularly, to an N-face GaN high electron mobility transistor (HEMT) device that includes an AlN or AlGaN buffer layer.
2. Discussion of the Related Art
Integrated circuits are typically fabricated by epitaxial fabrication processes that deposit or grow various semiconductor layers on a semiconductor substrate to provide the circuit components of the device. Substrates for integrated circuits include various semiconductor materials, such as silicon, InP, GaAs, etc. As integrated circuit fabrication techniques advance and become more complex, more circuit components are able to be fabricated on the substrate within the same area and be more closely spaced together. Further, these integrated circuit fabrication techniques allow the operating frequencies of the circuit to increase to very high frequencies, well into the GHz range.
HEMT devices are popular semiconductor devices that have many applications, especially high frequency or high speed applications. GaN HEMT devices are typically epitaxialy grown on a suitable substrate, such as silicon carbide (SiC), sapphire, silicon, etc., all well known to those skilled in the art. One fabrication process for GaN HEMT devices is referred to in the art as Ga-face fabrication, where the device profile layers are grown having a positive, or Ga polar orientation. For example, a typical HEMT device may have a SiC substrate that includes alternating crystalline layers of silicon and carbon. A nucleation layer, such as an AlN layer, is typically deposited on the SIC substrate to facilitate epitaxial growth, where the nucleation layer is grown on the side of the substrate having a silicon face so that the orientation of the crystalline structure of the nucleation layer, and the subsequent device layers, has a gallium orientation. When the gallium and nitrogen are provided to the vacuum chamber for the epitaxial deposition process of the nucleation layer, the semiconductor elements will be deposited on the substrate based on their crystal orientation so that alternating layers of gallium or aluminum and nitrogen are formed, where a nitrogen layer is formed first. A GaN buffer layer is typically grown on the nucleation layer that provides a crystalline structure having limited defects. An AlGaN barrier layer is deposited on the buffer layer, where the combination of the buffer layer and the barrier layer creates a two-dimensional electron gas (2-DEG) layer for the flow of electrons at the transition between these layers.
Various types of Ga-face fabrication processes have typically been employed because of the efficiency with which the epitaxial layers can be grown. It has been proposed in the art to reverse the orientation of the epitaxial growth process so that the opposite side of the substrate is the side on which the other device profile layers are grown, referred to as N-face or N-polar device processes. A typical N-face GaN HEMT device, sometimes referred to as an inverted-HEMT, typically includes an AlN nucleation layer, GaN buffer layer, an AlGaN back-barrier layer and a GaN channel layer. For the example discussed above for the SiC substrate, the device profile layers are grown on the carbon face of the substrate, so that the crystalline orientation of the device profile layers has a nitrogen orientation instead of a gallium orientation.
As discussed above, for an N-face device, similar layers are basically deposited as for a Ga-face device, but they have an opposite orientation and polarity so that the orientation of the aluminum, gallium and nitrogen crystals that make up the AlGaN/AlN nucleation layer, GaN buffer layer and GaN channel layer are opposite in crystalline orientation. The GaN channel layer is grown on the AlGaN back-barrier layer, where the 2-DEG channel is then formed between those two layers. The channel electrons in the 2-DEG layer are induced from the piezoelectric/spontaneous polarization effect between the AlGaN/AlN back-barrier and the GaN channel layer. For the N-face device, when the AlGaN back-barrier layer is grown on the GaN buffer layer, the opposite orientation of the crystals does not cause the 2-DEG layer to be formed therebetween.
Although N-face fabrication processes are typically more difficult than Ga-face fabrication processes, N-face fabrication processes typically provide more desirable results when forming the 2-DEG layer. Because the channel layer is formed on the AlGaN back-barrier layer in an N-face device, a number of advantages can be realized, such as the ability to make better electrical contact for the source and drain terminals. Also, because the channel layer is between the barrier layer and the contacts, the on/off switching of the device can be performed more quickly and efficiently. Further, the location of the channel layer reduces the buffer leakage current, which saves power and increases performance.
Vertically scaling of an N-face HEMT device can be performed to reduce the gate-to-channel distance, which causes a natural surface depletion to occur that reduces the charge of the 2-DEG layer. This requires a larger polarization charge to provide compensation. The thickness and/or aluminum composition of the AlGaN/AlN back-barrier layer must be increased to maintain sufficient carrier density of the 2-DEG layer. With a standard GaN buffer layer profile, increasing the aluminum composition or back-barrier thickness creates a large stress in the device as a result of differences in the atomic spacing. As a result, the wafer can crack or have severe bowing. This cracking can deplete the charge in the 2-DEG layer. Further, wafer bowing causes a low yield for high resolution lithography.